I am not sure there is any clear legitimate use for deep validation (beyond checking for syntax). If you are providing a valuable service to a customer, wether

I am not sure there is any clear legitimate use for deep validation (beyond checking for syntax). If you are providing a valuable service to a customer, wether it's a catch-all or free mail shouldn't matter to you. Now, if you are buying lists here and there to spam the hell out of it, the bounce rate would flag you very quick and you'd need to find another smtp provider every week. This service would be your life line.